Organisms existing in the natural world are exposed to a variety of environmental stresses such as salt, high temperature, low temperature, freezing, drying, and water stresses. Particularly, the salt stress is one of the biggest factors that inhibit the growth of many higher plants. Since the enhanced salt tolerance of higher plants leads to increase in the production of agricultural products, attempts to enhance the salt tolerance of higher plants by gene transfer have been made actively. Proteins having a function of enhancing salt tolerance can be divided broadly into compatible solute (glycinebetaine, mannitol, trehalose, or proline)-synthesizing enzymes, enzymes (Na+/H+ antiporters) participating in ion homeostasis and transcriptional regulators that regulate the expression of their genes. Many cases have been reported in recent years in which improvement in salt tolerance has been observed in transformed cells with the expression of these proteins. For example, there have been reported a plant of the genus Eucalyptus comprising a choline oxidase-encoding gene introduced in the chromosomal DNA (see Patent Document 1), a DNA encoding an Na+/H+ antiporter derived from a salt tolerance blue-green alga (Aphanothece halophytica) (see Patent Document 2), and a method for increasing a stress tolerance of a plant, characterized by suppressing the expression of a gene encoding an enzyme of the intracellular proline degradation system of the plant (see Patent Document 3). However, transformants that tolerate sea water containing approximately 500 mM NaCl have not been obtained yet for organisms (particularly, plants) with the expression of these factors enhancing salt tolerance. Only the compatible solute synthesis-system enzymes or enzymes participating in ion homeostasis seem to be insufficient for constructing transformants that tolerate sea water.
On the other hand, plants (halophytes), which have acquired a strong salt-tolerance mechanism during their evolutional processes, are present in the natural world. It has been expected that transformants (particularly, plants) that have acquired strong salt tolerance comparable to that of halophytes can be produced by using the salt tolerance factors of such plants alone or in combination of several of them. For example, the present inventors paid attention to mangrove, one of halophytes. The present inventors obtained cultured cells separated from the already-established cultured cell line of Bruguiera sexangula, and successfully enhanced the salt tolerance of yeast, a plant cell (cultured tobacco cell), and a plant (tobacco) by culturing this cell line in the presence of 100 mM NaCl, preparing a cDNA library on the basis of mRNA extracted from the cultured cells, attempting to search genes participating in the salt tolerance of mangrove from the library, and introducing one gene among the isolated genes participating in stress tolerance (see Patent Document 4). Moreover, the present inventors found a novel salt tolerance-increasing gene in Kandelia candel (L.) Druce, which is one species of mangrove, and determined the nucleotide sequence of the gene and amino acids encoded thereby. It has further been confirmed that the introduction of the gene into a different plant increases the salt tolerance of the different plant (see Patent Documents 5 and 6). However, findings on a salt tolerance factor derived from a halophyte are very few until now.
Patent Document 1: Japanese Laid-Open Patent Application No. 2003-143988
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-180373
Patent Document 3: Japanese Laid-Open Patent Application No. 2003-186879
Patent Document 4: Japanese Laid-Open Patent Application No. 2001-333784
Patent Document 5: Japanese Laid-Open Patent Application No. 2003-116546
Patent Document 6: Published Japanese translation of PCT international publication No. 2003-512837